The present invention relates to a method of measuring and controlling the level of an elevated temperature liquid in a container, and particularly to a method of measuring and controlling the level of molten metal in a vessel, for example, a mold in a continuous casting machine.
A system commonly used to measure the level of molten steel in a continuous caster mold utilizes a plurality of thermocouple probes located at spaced elevations on the mold wall. The probes provide electrical voltage signal outputs corresponding to the thermal profile along the mold wall due to the liquid steel in the mold. The conventional apparatus for determining the liquid level from the probe output signals is disclosed in U.S. Pat. No. 3,204,460, Milnes and U.S. Pat. No. 3,399,566, Wilson. The recorder balance slide-wire has worked adequately for many years. But recent changes in casting practices and more stringent process control requirements have established the need for changes in the equipment. Variation in the casting flux and flux buildup on the mold walls as well as submerged tube pouring have increased problems of determining the steel level in the mold. In addition, false level indications sometimes occur due to sudden mold level and casting speed variations. Perhaps the most common problems are false recorder oscillation and sluggish response due to improper amplifier gain adjustments. However, serious consequences may result from the problems of poor probe contact or shorted probes resulting in lower than normal or zero voltage signal outputs. The effect of these factors is indicated in FIGS. 1 and 2, respectively, where false levels are indicated (dotted arrows) instead of the true level (solid arrow).
It is the primary object of this invention to overcome the aforementioned difficulties associated with conventional thermoprobe liquid level measurement systems.